DiGi made the following startling claim in Omo's comments:

You know, I’ve had a copy of Dearly Stars since it’s release, and I’ve still not got around to playing it. Part of this is that it’s one of those annoying DSi-enhanced games that are region-locked (therefore, won’t play on my larger-screened EU DSes).

I must confess that I have no idea what he is talking about, because my Dearly Starts runs just fine on a U.S.-market DS:

The print on the box does not say a word about DSi, and if I remember right, the game came out before DSi. It does mention some mysterious Wi-Fi feature though...

Coincidentially, IM@S2 for PS3 works just fine on U.S. console too.

UPDATE:

Dearly Stars works fine on a regular DS, regardless of region. The problem is that, when Nintendo introduced the DSI (and the XL), they created something of a double standard – most DS games are region free, but any which use any of the additional functionality of the DSi will only work on consoles of the same region.

Dearly Stars as some additional functionality which can utilise the cameras on the DSi – something to do with scanning QR codes, as I recall – and as a result, non-Japanese DSi/DSiXL consoles refuse to so much as acknowledge that there’s anything in the cartridge socket. I mean, it’s not like I’ve not got a regular DS (both Lite and Phat), but the screens are a good degree smaller than on a DSi or even a 3DS. It’s kind of hard to go back at this point.